Apacs Stern 100 Badminton Racket (4U) Isomectric head frame - Equalizes the length of main and cross strings in the stringbed, enlarging the sweet-spot for more consistent accuracy even on off-center hits. Reinforced Structure Frame - The new material is adopted in the frame in order to make the frame Ultra high Strength & Stiffness. As a result it can handle string tension of up to 35lbs. Nanotech Carbon - The new material 'Nanotech Carbon' is adopted in both frame and shaft to give strength and perseverance. The strength and flexibility that enhances the racket Power and Speed will lead your game into the next level.